This invention is related generally to accessories for sheet-fed, rotary offset printing presses, and in particular to a dryer for printed materials which utilizes infra-red radiant heat, forced air flow and extraction.
In the operation of a rotary offset press, an image is reproduced on a sheet of paper or some other print stock by a plate cylinder which carries the image, a blanket cylinder which has an ink transfer surface for receiving the inked image, and an impression cylinder which presses the paper against the blanket cylinder so that the inked image is transferred to the paper. In some applications, a protective and/or decorative coating is applied to the surface of the freshly printed sheets. The freshly printed sheets are then conveyed to a sheet delivery stacker in which the finally printed sheets are collected and stacked.
The wet ink and coatings should be dried before the sheets are stacked or run back through the press for a second pass, to prevent smearing defects and to prevent offsetting of the ink on the unprinted side of the sheets as they are stacked. Spray powder has been applied between the freshly printed sheets which are to be stacked to improve sheet handling and to separate one delivered sheet from the next sheet to prevent offsetting while the ink and/or coating dries. One limitation on the use of spray powder is that fugitive particles of the spray powder disperse into the press room and collect on press equipment, causing electrical and mechanical breakdowns and imposing a potential health hazard for press room personnel.
Hot air convection heaters and radiant heaters have been employed to reduce the volume of spray powder applied, except for the small amount needed for sheet handling purposes. Hot air convection heaters are best suited for slow to moderate speed press runs in which the exposure time of each printed sheet to the hot air convection flow is long enough that aqueous base inks and coatings are set before the sheets reach the stacker.
For high-speed press operation, for example, at 5,000 sheets per hour or more, the exposure time of each printed sheet as it passes through the dryer station is not sufficient to obtain good drying by convection flow alone. Radiant heaters such as infra-red heat lamps provide greater drying efficiency because the short wave length infra-red energy is preferentially absorbed in the liquid inks and coatings to provide rapid evaporation. The infra-red radiant energy releases water and volatiles from the ink and/or coating. Consequently, a humid air layer clings to the printed surface of the sheet as it moves through the dryer, and will be trapped between adjacent sheets in the stack unless it is removed.
As press speed is increased, the exposure time (the length of time that printed sheet is exposed to the radiant heat) is reduced. Consequently, the output power of the radiant lamp dryers has been increased to deliver more radiant energy to the printed sheets in an effort to compensate for the reduction in exposure time.
The higher operating temperatures of the high-powered lamps cause significant heat transfer to the associated printing unit, coater and press frame equipment, accelerated wear of bearings and alterations in the viscosities of the ink and coating, as well as upsetting the water balance of aqueous coatings. The heat build-up may also cause operator discomfort and injury.
The principal object of the present invention is to increase the operating efficiency of a printing press dryer of the type which utilizes radiant lamps to dry inks and coatings on freshly printed and/or coated sheets.
A related object of the present invention is to provide a high efficiency, high power output radiant heater which includes improved means for limiting the transfer of heat to nearby parts and press equipment.
Another object of the present invention is to increase the effective exposure time of a freshly printed sheet to forced air flow in a printing press dryer so that the printing press may be operated at higher speeds without compromising quality.
Yet another object of the present invention is to provide an improved radiant heat dryer of the character described which includes means for removing the humid air layer from the surface of a freshly printed sheet and extracting it from the press, thereby accelerating the drying process.
The foregoing objects are achieved according to the present invention by a combination forced air and radiant heat dryer in which the exposure to forced air flow is increased by broadening the air base. Forced air at high pressure is discharged uniformly through precision holes located directly above an array of infra-red lamps onto a freshly printed and/or coated sheet as it moves along a sheet transport path to a delivery stack.
According to one aspect of the present invention, the moist air layer is displaced from the surface of the printed sheet by high-velocity air jets which scrub and break-up the moisture-laden air layer that adheres to the printed surface of the sheet. The high-velocity air jets create turbulence which overcomes the surface tension of the moisture and separates the moisture laden air from the surface of the paper. The moisture laden air becomes entrained in the forced air flow and is removed from the press as the moisture laden air is extracted.
Effective exposure to the forced air flow is increased by multiple air jets, in which the air jets are arranged to deliver a substantially uniform blanket of the high velocity air across the sheet transport path. Preferably, the high velocity air jets are uniformly spaced with respect to each other along the sheet delivery path. Since the release of moisture and other volatiles from the ink and/or coating occurs continuously during exposure in response to the absorption of infra-red radiation, the moisture laden air layer is displaced continuously from the printed sheet as the printed sheet travels through the dryer and crosses the multiple air jets.
After a printed sheet exits the dryer, and before the arrival of the next successive printed sheet, residual moisture-laden air is completely exhausted from the press by an extractor. According to this arrangement, the drying of each printed sheet is accelerated before it is placed on the delivery stack. If a protective coating is applied over the ink, the coating is completely dried and a dry film is established over the wet ink. This permits the ink to thoroughly cure under the coating after stacking, thus eliminating the need for spray powder to control offsetting.
Operational features and advantages of the present invention will be understood by those skilled in the art upon reading the detailed description which follows with reference to the attached drawings.